Laboratory of Cell Cycle Genetics, The Rockefeller University, New York, New York, United States of America.
PLoS One. 2011;6(6):e20788. doi: 10.1371/journal.pone.0020788. Epub 2011 Jun 10.
Diverse mitotic events can be triggered in the correct order and time by a single cyclin-CDK. A single regulator could confer order and timing on multiple events if later events require higher cyclin-CDK than earlier events, so that gradually rising cyclin-CDK levels can sequentially trigger responsive events: the "quantitative model" of ordering.
METHODOLOGY/PRINCIPAL FINDINGS: This 'quantitative model' makes predictions for the effect of locking cyclin at fixed levels for a protracted period: at low cyclin levels, early events should occur rapidly, while late events should be slow, defective, or highly variable (depending on threshold mechanism). We titrated the budding yeast mitotic cyclin Clb2 within its endogenous expression range to a stable, fixed level and measured time to occurrence of three mitotic events: growth depolarization, spindle formation, and spindle elongation, as a function of fixed Clb2 level. These events require increasingly more Clb2 according to their normal order of occurrence. Events occur efficiently and with low variability at fixed Clb2 levels similar to those observed when the events normally occur. A second prediction of the model is that increasing the rate of cyclin accumulation should globally advance timing of all events. Moderate (<2-fold) overexpression of Clb2 accelerates all events of mitosis, resulting in consistently rapid sequential cell cycles. However, this moderate overexpression also causes a significant frequency of premature mitoses leading to inviability, suggesting that Clb2 expression level is optimized to balance the fitness costs of variability and catastrophe.
CONCLUSIONS/SIGNIFICANCE: We conclude that mitotic events are regulated by discrete cyclin-CDK thresholds. These thresholds are sequentially triggered as cyclin increases, yielding reliable order and timing. In many biological processes a graded input must be translated into discrete outputs. In such systems, expression of the central regulator is likely to be tuned to an optimum level, as we observe here for Clb2.
单一的细胞周期蛋白激酶（cyclin-CDK）可以按正确的顺序和时间触发多种有丝分裂事件。如果后续事件需要比早期事件更高的 cyclin-CDK，那么单一的调节剂就可以为多个事件提供秩序和时间，从而使 cyclin-CDK 水平逐渐升高，从而依次触发相应的事件：这就是“定量模型”的排序原理。
方法/主要发现：该“定量模型”对固定水平的 cyclin 锁定延长时间的效果做出了预测：在 cyclin 水平较低时，早期事件应该迅速发生，而晚期事件则应缓慢、有缺陷或高度可变（取决于阈值机制）。我们在酵母细胞周期蛋白 Clb2 的内源性表达范围内，将其滴定到一个稳定的固定水平，并测量了三个有丝分裂事件（生长去极化、纺锤体形成和纺锤体伸长）发生的时间，作为固定 Clb2 水平的函数。这些事件按照正常的发生顺序，需要越来越多的 Clb2。在固定 Clb2 水平下，事件以类似于正常发生时的效率和低变异性发生。该模型的第二个预测是，增加 cyclin 积累的速度应该会全局提前所有事件的时间。Clb2 的适度（<2 倍）过表达加速了有丝分裂的所有事件，导致连续快速的细胞周期。然而，这种适度的过表达也会导致明显频率的过早有丝分裂，导致无法生存，这表明 Clb2 的表达水平被优化以平衡变异性和灾难的适应性成本。
结论/意义：我们得出结论，有丝分裂事件受离散的 cyclin-CDK 阈值调节。随着 cyclin 的增加，这些阈值依次被触发，从而产生可靠的顺序和时间。在许多生物过程中，必须将分级输入转化为离散输出。在这样的系统中，中央调节剂的表达可能会被调谐到最佳水平，正如我们在这里观察到的 Clb2 一样。
